Typical power amplifiers are constructed by interconnecting multiple "gain blocks" as shown in FIG. 1. These gain blocks are generally made up of substrates 104 and 108 that straddle the component (in this case a power transistor 106). More specifically, substrates 104 and 108, containing other component parts (not shown) are mounted onto a metallic plate 102. The metallic plate 102 is usually formed of a metal or alloy that has good electrical conductivity and heat transfer characteristics while the substrates 104 and 108 are typically made of a ceramic, or printed-circuit-board (PCB) type material. The transistor 106 is then mounted onto the metallic plate 102 in such a way as to be straddled by the substrates 104 and 108. The substrates 104 and 108 are connected to the transistor 106 via connector elements 110 and 112 that are soldered onto the substrates 104 and 108, with the metallic plate 102 acting as a ground plane for the device. In addition to serving as a ground plane, the metallic plate 102 also serves to dissipate the heat generated by the transistor 106.
One problem that exists with the current construction of power amplifiers in this gain block manner is there exists a mismatch in the coefficient of thermal expansion (CTE) between the substrates and the metallic plate. Because of this mismatch, the bonding area between the two needs to be quite small. This results in several problems: (1) the substrates are left unsupported over most of the metallic plate, which leads to a weak mechanical assembly, (2) the grounding of the substrate is, in most conditions, inadequate and somewhat unreliable, and (3) expensive bonding agents need to be utilized to accommodate for the mismatch in CTE. In addition to the problems with matching the CTE between the substrates and the metallic plate, the use of a bulky metal plate results in high material costs as well as a very heavy final part.
Thus a need exists for an apparatus which can effectively dissipate heat in a transistor, but has a low weight, eliminates the need to match CTEs, and is inexpensive to produce.